Results of research carried out at the U.S. Department of Energy’s (DOE’s) Advanced Photon Source (APS) may pave the way to improvements in industrial processes based on solvent extraction, which is used in the mining and refinement of technologically important rare earths. The results were published in the journal Physical Review Letters.
Rare earths such as lanthanides, which are elements in the range of atomic number 57 to 71, are not actually rare. They exist in large quantities in the world, but are only found in the form of trace amounts in rocks. Since rare earths are important for a variety of applications (e.g., electronics) their extraction is a major mining-related industry.
A common process by which rare earths are extracted involves dissolving rocks in acids, then shaking up the solution with an organic solvent and a surfactant. Under the right conditions, the desired ions move out of the aqueous phase and into the organic solvent. This is known as “liquid-liquid extraction” or “solvent extraction,” and is conducted on a large scale by the mining industry. This process also separates heavier lanthanides from lighter lanthanides present in the same solution, because the heavier lanthanides separate more easily. While this fact is known and exploited in industrial separations processes, the nanoscale mechanisms of the separation process are not well understood.
Image: (a ) Schematic of system studied; positively charged lanthanide ions (blue circles) dissolve in the water, while the negatively charged surfactant molecules (purple) float on the water surface. (b) Data showing how density of ions at the surfactant surface jumps as the concentration of ions in the bulk water increases (Er=erbium, a heavier lanthanide, Nd=neodymium, a lighter lanthanide). The lines thru data are predictions from computer simulations. From M. Miller et al., Phys. Rev. Lett. 122, 058001 (2019).